Wireless cellular networks enable communication among mobile devices such as cellular telephones, Voice Over Internet Protocol (VoIP) phones, wireless Personal Digital Assistants (PDAs), laptop computers, and other devices such as Personal Computers (PCs).
Communication with a wireless device is facilitated via a number of base stations or Access Points (APs) that are located in the current vicinity of the wireless device. As the location of the wireless device changes, a most-appropriate access point is chosen to handle data traffic on a forward link directed to the device and a reverse link received from the wireless device.
A hard handoff refers to a cellular network that makes the AP selection for handling the reverse link traffic in a relatively abrupt manner. A soft handoff refers to a cellular network that makes the AP selection in a relatively gradual manner. Soft handoffs can increase cell coverage and cell capacity.
In a soft handoff, M access points receive reverse link frame traffic from the wireless device and forward the frame traffic to a Selection Distribution Unit (SDU). The access points also forward frame quality indicators to the SDU, such as a Cyclic Redundancy Check (CRC), error flags, and signal strength readings. A Frame Selector (FS) in the SDU selects the best frame out of the M frames received from the M different APs for forwarding to higher levels of the network. This allows the SDU to use reverse link traffic from different APs in a non-abrupt and graceful manner.
Implementing soft handoffs require the APs to transmit multiple versions of the same frame over the backhaul (reverse link) cellular network to the FS. This unfortunately increases backhaul bandwidth and cost.